1. Field of the Invention
This invention pertains generally to control systems and, more particularly, to such systems employing rectifiers, such as three-phase semiconductor controlled rectifier (SCR) bridges, for converting alternating current (AC) to direct current (DC). The invention also pertains to a method for controlling and monitoring a parallel array of rectifier bridges.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle.
It is not uncommon for a plurality of SCR bridges to be operated in parallel with each of the corresponding bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the same point in the AC waveform.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous motors and generators, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs). For example, when the generator is on-line, generator field excitation is provided thereto.
As shown in FIG. 1, two controlled rectifier bridges 2,4 are connected in parallel. Each of the bridges 2,4 receives a multi-phase current input from a common AC source 6 (e.g., a motor generator, field transformer, power potential transformer (PPT)). See, for example, U.S. Pat. No. 6,232,751. A three-phase current input is employed in this example, although the invention is applicable to a wide range of phase counts. The several phases of input current, shown as 8A, 8B and 8C, are fed through contacts 10A, 10B and 10C, and 14A, 14B and 14C, respectively, to rectifying segments on the two respective bridges 2,4.
The exemplary bridges 2,4 each have six cells or segments, one for each polarity of each of the phase currents 8A,8B,8C, although the invention is applicable to a wide range of segment counts. For example, segment AP1 refers to the positive polarity of phase A in the first bridge 2, while segment AN1 refers to the negative polarity of phase A in that first bridge 2. The remaining segments of the first and second bridges 2,4 are noted in a similar manner by reference characters BP1,BN1,CP1,CN1 and AP2,AN2,BP2,BN2,CP2,CN2.
Each bridge segment includes its own element, such as the exemplary SCR 12, that has a firing input 15. When a suitable control signal is provided to one of the inputs 15, the corresponding SCR fires to, thereby, control current conduction (e.g., by starting or initiating current conduction) within the corresponding segment. Typically, the individual segments of each of the bridges 2,4 are fired every 60° of the AC cycle in the order: APn,CNn,BPn,ANn,CPn,BNn (where, for convenience of reference, APn, for example, refers to either AP1 or AP2). One of the SCRs 12 does not stop conducting until it is reversed biased. All of the positive rectifier output currents are summed together and the negative rectifier output currents are similarly summed and conveyed by corresponding positive and negative conductors 16,18 to a load (not shown).
An exemplary microprocessor-based control and monitoring circuit 20 has six control outputs 22, which are interconnected with the six firing inputs 15 of the SCRs 12, for outputting control signals to the segments AP1,BP1,CP1,AN1,BN1,CN1 to control current conduction within those segments, and various monitoring inputs 24,26. The six control outputs 22 include six digital logic control signals each of which controls current conduction within a corresponding one of the segments. The circuit 20 employs a firing code that has six bits. Each of the six bits is set when a corresponding segment is to fire (e.g., bit 0 for APn, bit 1 for BPn, bit 2 for CPn, bit 3 for ANn, bit 4 for BNn, and bit 5 for CNn). Each time a firing occurs, two cells are fired. Also, a cell is usually fired a second time in order that the second firing occurs in the next subsequent firing (e.g., by first firing APn and CNn with the firing code=1000012, followed by firing CNn and BPn with the firing code=1000102).
Monitors 28A,28B,28C provide conduction monitor signals to the inputs 26 for the three AC phases A,B,C. The monitors 28A,28B,28C monitor the respective phases A,B,C to provide corresponding conduction signals. The control and monitoring circuits 20 also input current signals from the shunts 30,32 and use that information to control the output of the respective bridges 2,4 through DC contact actuators (not shown).
The conduction monitors 28A,28B,28C typically provide not conducting, conducting positive, conducting negative, conducting negative and positive, and/or failed signals. For example, U.S. Pat. No. 5,963,441 discloses conduction monitors which output conduction monitor signals as four-state logic signals having a first state (e.g., 102) representative of the positive polarity of AC input current; a second state (e.g., 012) representative of the negative polarity of the AC input current; a third state (e.g., 002) representative of about zero AC input current; and a fourth state (e.g., 112) representative of failure of the monitor.
Additional or different hardware and/or software provides information that the conduction monitor detects both conducting negative and positive. For example, each of the conduction monitors 28A,28B,28C, as shown with the conduction monitor 28C, includes a forward (F) current sensor 28CP for the corresponding element CP1 and a reverse (R) current sensor 28CN for the corresponding element CN1. The conduction monitors 28A,28B,28C provide feedback to the control and monitoring circuit 20 (e.g., excitation control regulator) that the corresponding SCR bridge 2 is properly conducting.
In series with the contacts 10A,10B,10C,14A,14B,14C, or in place of such contacts, six fuses (not shown) may be employed. Also, for each of the bridges 2,4, a di/dt inductor (not shown) may be electrically connected in series with each of the SCRs 12.
If the SCR bridges 2,4 are employed in a generator excitation control system (not shown), then the generator (not shown) includes an output having three AC phases (not shown). A three-phase PPT transformer (not shown) includes three primary windings in a WYE-configuration and three corresponding secondary windings in a delta-configuration, although other transformer configurations may be employed. The primary windings are interconnected with the generator AC phases, while the corresponding secondary windings have three AC phases, each of which has a positive polarity and a negative polarity and a common frequency (e.g., 50 Hz, 60 Hz, 420 Hz), which form the common AC source 6. The PPT is normally connected to the terminals of the generator and, thus, the system is commonly referred to as a “terminal fed excitation system”.
For static excitation systems, the PPT is electrically connected to a suitably reliable power source. In some systems, this is a station service source and in others it is the generator terminals. Most designs for PPTs will accommodate a certain amount of unbalanced operation. However, significant unbalance will saturate the PPT's core, thereby causing high operation temperatures. If this significant unbalance continues, then damage to the PPT may result.
U.S. Pat. No. 5,963,441 discloses an algorithm, which obtains individual SCR current measurements. The current is decremented when an SCR was fired twice, but was not conducting.
There are known separate hardware devices (e.g., current differential and voltage relays) that can be added to excitation equipment, in order to detect various unbalanced operating conditions. Although such devices can provide the desired protection, they are separate, add cost and must be mounted in the equipment.
Accordingly, there is room for improvement in control systems and methods for controlling and monitoring a parallel array of rectifier bridges.